THE ANATOMY OF THE THYROID GLAND 153 



desired, though the organ has been much studied in this class of 

 animals. 



In Aves the thyroid gland has been extensively studied by 

 Simon ('44), Peremischko ('67) who considered the histology 

 as well as the gross anatomy of the organ, Baber ('81) and De 

 Meuron ('86). The literature of the structure and development 

 of the thyroid in the chick is extensive. 



In most of the Mammalia the anatomy of the thyroid gland is 

 well known and its literature has acquired voluminous proportions. 

 Its review does not fall within the scope of the present paper. 



REVIEW OF THE LITERATURE 



A careful study of the available literature has revealed, with the 

 exception of the work of Guiard, only casual references to the 

 anatomy of the thyroid gland in the Elasmobranchs. Simon 

 ('44) studied the Selachian (Squalus) and the skate (Raia). He 

 describes the thyroid gland as "a single organ, situated in the me- 

 dian line, in connection with the anterior surface of the cartilages 

 which bind together the branchial arches of opposite sides of the 

 body," and he states that it may lie in contact with the ''lingual 

 bone," or may be more or less distant from the mouth, but that 

 it is ''always at the spot where the great trunk of the branchial 

 aorta distributes its terminal branches. It lies at the angle of 

 this bifurcation . . . ; it is covered by the sterno-hyoid 

 or sterno-maxillary muscle, and also by the myo-hyoid and genio- 

 hyoid, w^hen these are present." His description I find to hold 

 good for Raia, but it does not entirely correspond to the position 

 of the thyroid gland of Squalus, Mustelus, or Carcharias. As to 

 its vascularization, Simon states that the gland receives its blood 

 supply by means of a recurrent branch given off by the first 

 branchial vein, while yet within the gill, and that "it never 

 receives the smallest share of supply from the branchial artery 

 with which it is in contact." The last portion of this statement 

 is precisely correct for all the species which I have examined, 

 though apparently at variance with the observations of some 

 authors : the first portion, as to the origin of the thyroid artery, 



